Vibrating screen

ABSTRACT

A vibrating screen for the sizing of granular material such as gravel, sand, crushed stone, etc., having a frame (1) and a screen body (2) supported on springs (6), directional oscillating movements being imparted to the screen body by a motor powered vibrator mechanism (3). The screen has two or more screen decks (11-13), each one divided into three component screens (14a-c, 15a-c, 16a-c) having successively decreasing inclinations in a direction towards the discharge end (9) of the screen, each lower screen deck furthermore having an increased inclination in relation to the nearest deck above.

This invention relates to improvements in a known type of vibratingscreen for sizing granular materials such as gravel, sand, crushedstone, etc., in which an oblong screen body is movable in relation to aframe by a motor powered vibrator mechanism which is connected to thescreen body. In these known screens, oscillating movements are impartedto the screen body by the vibrator mechanism; there is an upper screendeck and one or more lower screen decks; there are successivelydecreasing screen openings for each lower deck; the decks are inclineddownward from the feed end toward the discharge end; each screen deck iscomposed of a plurality of component screens arranged one after theother; each component screen is provided with a screening element suchas metal wire cloth or netting; and, the component screens are arrangedwith successively decreasing inclinations relative to the horizontalplane.

OBJECT OF THE INVENTION

The objects of the present invention are to obtain a vibrating screenwhich, under limitation of its dimensions and weight, has a highcapacity and makes it possible to produce a plurality of accuratelyseparated product fractions and which also, due to the mentionedlimitations, has a low acquisition cost and is easy to transport.

In brief, the stated objects have been attained by fitting the vibratingscreen in accordance with the invention with a vibrator mechanism with adirectional throw in combination with a specific arrangement of theinclination of the different screen decks, in addition to which awidth-saving tensioning of the screen cloths or nettings is madepossible by means of a specific tensioning arrangement.

BACKGROUND

In crushing and screening plants, for example for the production ofconcrete ballast or asphalt material, the development has gone towardsincreasing capacity demands in combination with demands for theproduction of accurately sized, short product fractions, i.e. fractionshaving a small span between the upper and lower fraction limits. Atypical example of such fractions, and of an application of thevibrating screen in accordance with the present invention, is thedividing of a feed material 0-16 mm into the fractions 0-4, 4-8, 8-11.2and 11.2-16 mm. An additional typical prerequisite is that the capacityof the screen should be at least 150-200 metric tons per hour and,furthermore, that it should be easily portable in order to make itpossible also to exploit smaller material deposits in an economical way.For the last-mentioned prerequisite, in addition to the transportabilityand, thereby, the transport cost, the acquisition cost of the screen isalso of importance.

Even discounting the demand for transportability, it is a greatadvantage to be able to keep the dimensions of the screen as small aspossible. Large and heavy screens with large vibrating masses implysevere strains on the screen body and require a meticulously correctdimensioning and balancing of same. Furthermore, heavily dimensionedbearings are required for the vibrator mechanism, which bearings cannotendure a high speed, i.e. a high stroke frequency of the vibratingmovement. The acceleration or throwing effect on the material to bescreened is thereby impaired. Furthermore, the screens are of coursemore expensive in acquisition and require more energy for theiroperation.

A typical example of a screen in accordance with the invention which isadapted to the above-mentioned production prerequisites is a multideckscreen having an effective width of the screen decks of 1800 mm and aneffective length of same of 3300 mm. Even larger width and lengthmeasures can come into the question for the obtaining of highercapacities.

It is known earlier to divide a screen deck into a plurality ofcomponent screens having a successively decreasing inclination in thedirection towards the discharge end of the screen. This arrangementresults in different transport speeds of the material to be screenedalong the different component screens, which is advantageous for thescreening efficiency. A fast moving away of the material to be screenedis required at the first portion of the screen deck where it has tohandle a large amount of material, as otherwise the bed of material willbe too thick for the undersize particles in its top layer to be screenedthrough. Towards the middle and final portions of the deck, the bed ofmaterial is thinned out, and the transport speed can be lowered so thatthe material particles are thrown up and fall down a large number oftimes during their transport, the undersize particles thereby gettingincreased chances of passing through the screen openings. Withsucessively reduced inclinations, the total height of the screen is, inaddition, reduced, the total length of the screen decks at the same timebeing maintained. In the vibrating screen in accordance with theinvention, this deck arrangement has been utilized in a partly new way,as will be described more closely hereafter.

The vibrating screen in accordance with the invention is in each screendeck fitted with three component screens having a successivelydecreasing inclination. For the tensioning of the screeningelements--which can be for example metal wire cloths, wire nettings orplastic cloths provided with apertures--end tensioning has been chosen,i.e. they are tensioned in the longitudinal direction of the screen.This results in the smallest possible loss of effective internal width,so that the width of the screen can be kept down. It also makes itpossible to use long-mesh wire nettings or cloths, i.e. cloths havingoblong, rectangular screen openings which have a width at right anglesto the feed direction which corresponds to the size of the desiredproduct fraction, but which have a length considerably greater than thesize of the fraction. Side tensioning, which is another commontensioning method, results in a considerable loss of effective width,since space-requiring tensioning elements are added inside the sideplates of the screen, and would therefore require an increased totalwidth to obtain the required effective width. In addition, sidetensioning makes it difficult or impossible to use long-mesh screencloths, the wires to be tensioned in the lateral direction being too fewand spaced too widely apart to give the cloth the required firmness.

An end-tensioned deck with two component screens constitutes noparticular problem in connection with the replacement and tensioning ofthe screening elements, since these are accessible from each end of thescreen body. For the arrangement chosen in accordance with theinvention, however, the middle one of the three component screens ineach deck constitutes a problem which is solved with the help of aspecific device which is described in closer detail hereafter.

DESCRIPTION OF THE ATTACHED DRAWINGS

FIG. 1 shows a side view of a vibrating screen in accordance with theinvention which is supported on a frame and fitted with slides or guidechutes for collecting the material fractions produced.

FIG. 2 shows a strongly diagrammmatic, partly sectioned side view of thescreen body only, without frame and chutes.

FIG. 3 is a partial enlargement, comprising the infeed end of thescreen, of the view in accordance with FIG. 2.

For the sake of lucidity, a number of parts have been left out which arenot essential for the understanding of the invention, such as supportframes for supporting the screen decks, seals between the screeningelements and the sides of the screen body, stiffeners, etc.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The vibrating screen in accordance with the invention comprises a frameor stand 1, a screen body 2 and a vibrator mechanism 3 fitted on sameand having an electric motor 4 and a V-belt drive 5. The mechanism is ofthe twin-shaft type having off-centre weights fitted on the shafts whichweights, in a known way, co-operate in two opposite directions andcounteract each other in all other directions, thereby producing amainly linear reciprocating movement. This movement is transmitted tothe free-swinging screen body which is supported on springs 6. Thesprings are supported on the frame 1.

The screen in accordance with the embodiment of the invention shown inFIG. 1 is driven by two motors and two V-belt drives driving one each ofthe two vibrator shafts. In the figure, however, only one motor andV-belt drive are visible, the other pair being obscured. When the screenis driven in this way, the movements of the two vibrator shafts areautomatically synchronized by the effect of the off-centre weights. Itis also possible to use only one motor to drive one of the vibratorshafts which by means of a spur gearing drives the other shaft.

At the feed end 7 of the screen body, a feed plate 8 without screenopenings is provided for the reception and distribution of the feedmaterial. The opposite end of the screen body, the discharge end, isdesignated by the numeral 9. The screen is provided with three screendecks 11-13 arranged one above the other and each one consisting ofthree component screens 14a-c, 15a-c, 16a-c. Each component screencomprises a screening element 17-19, not shown in detail, such as ametal wire cloth or netting provided with meshes, said screeningelements being fitted with hook strips 20, 21 at both ends. The hookstrips are at one end of the screening element hooked on to fixedholders 22 and, at the opposite end, to movable tensioning irons 23 and24, respectively. In FIG. 2 only a few of the just mentioned parts 20-24have been designated by numerals, as otherwise the figure would becometoo confused.

The vibrator mechanism 3 is protected from the screened material fallingdown by an unperforated plate 25 which leads the material passingthrough tne bottom screen deck to a chute or slide 26. The othermaterial fractions are collected and led away by chutes or slides 27-29.

The chosen end tensioning of the screening elements presents a problemas regards the middle component screen in each deck. The componentscreens 14a, 14c, 15a, 15c, 16a, 16c, which are directly accessible fromthe ends of the screen body, can be fitted with tensioning irons 23which in direct connection are provided with tensioning screws 30 havingnuts 31. The tensioning screws can be so located that they provide for adirection of pull which corresponds to the desired tensioning directionof the screening element, i.e. to its inclination relative to thehorizontal plane. For the middle screens 14b, 15b, 16b in each deck, thetensioning of the screening elements cannot be carried out in the samesimple way. To solve the problem of tensioning these screening elements,the tensioning screws for same have therefore, in accordance with theinvention, been moved to the feed end of the screen body where they areeasily accessible, and are in each deck located below the tensioningscrews of the outer component screen in the same deck, i.e. in the spacebetween these screws and the tensioning screws of the nearest deckbelow. This means that the point of operation of the tensioning deviceat the end of the deck is not at the correct height to correspond to thedesired tensioning direction. For the middle screening elements,tensioning screws 32 are therefore provided, each one being flexiblyconnected to a tensioning slide 35 by means of an extension rod 34. Theslide is provided with guide irons 36 which are guided in the desiredtensioning direction by guides 37, 38 welded to the side of the screenbody. The movable tensioning iron 24, which extends substantially acrossthe total width of the component screen, is at least at each one of itstwo ends fixedly connected to a tensioning slide 35 and is hooked on tothe tensioning strip 20 of the screening element. The point ofarticulation at which the pulling force acts on the tensioning slide islocated a little lower than the point of engagement between thetensioning iron 24 and the tensioning strip 20. This is in order toobtain a force which strives to turn the tensioning slide in such a waythat its rear end moves downwards and its front end upwards. The turningforce causes the guide iron 36 of the tensioning slide to press againstthe guides 37, 38, so that the vibrating movements of the screen cannotmake the guide iron hammer against the guides and cause a successivelyarising play.

At the feed end of the vibrating screen, covers 40 are provided in orderto prevent spillage of material through it, which covers can be removedto provide accessibility for the replacement of screening elements. Thecovers also support spillage protection plates 41 which prevent thespillage of material down to the deck below between the covers and theends of the screening elements. The covers are clamped in place byretaining irons 42 which are secured by screws 43 provided with nuts.

The three component screens in each screen deck are arranged with aninclination which successively decreases towards the discharge end ofthe screen. Furthermore, the arrangement is such that each componentscreen in a lower deck is somewhat more steeply inclined than thecorresponding component screen in the next deck above it, i.e. thecomponent screen which is located substantially vertically above thecomponent screen of the lower deck. The different inclinations have beenchosen with regard to obtaining for each deck the optimum thickness ofthe bed of material and the optimum transport speed of it towards thedischarge end of the deck in order to obtain, thereby, at all pointsalong the decks, a high processing capacity while retaining a highscreening efficiency. The target of the choice of inclinations is toobtain, at the beginning of each deck, a bed thickness of approximatelytwice the screen opening size, which is a suitable bed thickness for theattaining of both a high capacity and a good screening efficiency, andto maintain also for the following component screens a bed thicknesscorresponding to or slighty lower than the just mentioned measure. Thisis achieved thereby that the chosen inclinations provide for a correctlyadjusted transport and distribution of the bed of material at all pointsalong the decks in relation to the quantities of material and the sizesof the screen openings at the respective points.

In combination with this arrangement, the vibrating screen isfurthermore, in order to obtain a controlled feeding movement of thematerial to be screened, provided with a vibrator mechanism of the typewhich produces a substantially linear reciprocating movement resultingin a throwing effect on the material particles directed obliquelyupwards and forwards in a direction towards the discharge end of thescreen. The predetermined direction of throw in combination with thechoice of a suitable frequency and amplitude of the movement produced bythe vibrator mechanism makes it possible to calculate and maintain anoptimum transport speed of the material in relation to the inclinationsof the decks, so that the material particles are lifted up and lowered asufficient number of times for the undersize particles to be caught bythe screen openings and pass through them.

In order to increase the capacity of the screen even further whilemaintaining an accurate sizing, the component screens of each screendeck are arranged in steps, i.e. the end portions of adjoining componentscreens are located at a vertical distance from each other. The materialbed transported along a screen deck is thereby subjected to a turnovermovement when passing from one component screen to the next-followingone. In order to eliminate the risk of material passing through the gapbetween the edges of the component screens, the component screensoverlap each other a distance in the horizontal plane.

Of the three component screens in each deck, the last one--designatedc--is longer than the two other screens. It is therefore, in order to beheld in place without "flapping", tensioned to form an arch over supportirons 10. Such support irons are not shown for the shorter componentscreens, but can come into the question for these, too, depending on howlong they are made. By dividing the first half of the screen decks, asshown, into two component screens with different inclinations, a quickdistribution and coarse sizing of the material is obtained on the firstscreen, on the second one a slightly reduced transport speed and a moreaccurate screening out of particles from the now thinned-out bed ofmaterial and on the third, long component screen a still lower transportspeed and the final sizing of the material. This three-stage screeningin combination with the choice of different lengths of the componentscreens provides for a considerably higher capacity, while maintaining agood screening efficiency, than if the deck should only be divided intotwo component screens with different inclinations or, alternatively,have the same inclination all the way.

The embodiment of the invention shown and described is only an example,and variations of the design are possible within the scope of theclaims.

What is claimed is:
 1. A vibrating screen for the sizing of granularmaterial, comprising a frame (1) and an oblong screen body (2) which ismovable in relation to said frame, oscillating movements to be impartedto said screen body by means of a motor powered vibrator mechanism (3)connected to said screen body, said screen body being at one end, thefeed end (7), arranged to receive unscreened material and furthermorebeing provided with an upper screen deck and one or more underneath saidupper deck located lower screen decks (11-13) provided with successivelydecreasing screen openings for each lower deck, the decks being inclineddownwards from said feed end (7) towards an opposite end of said screenbody, the discharge end (9), and each screen deck being composed ofthree or more component screens (14a-c, 15a-c, 16a-c) arranged one afterthe other and provided with screening elements (17-18), said componentscreens being arranged with successively decreasing inclinationsrelative to the horizontal plane, said vibrator mechanism (3) beingarranged to impart to said screen body (2) directional oscillatingmovements which impart to said material lifting or throwing movementsobliquely forwards and upwards in a direction towards said discharge end(9), at least one said component screen in a lower screen deck having agreater inclination in relation to the corresponding component screen ofthe nearest deck above.
 2. A vibrating screen in accordance with claim1, wherein said component screen (14c, 15c, 16c) which in each of saidscreen decks (11-13) is closest to said discharge end (9), is longerthan the other component screens (14a-b, 15a-b, 16a-b) in said deck. 3.A vibrating screen in accordance with claim 2, wherein the componentscreen which is closest to said discharge end is at least 50% longerthan the other component screens in the respective deck.
 4. A vibratingscreen in accordance with claim 1, wherein all said screening elements(17-19) of said screen decks are tensioned in the longitudinal directionof said oblong screen body (2), said screen decks including middlescreening components which are spaced from the feed and discharge endsof the screen body, said screening element of a middle screeningcomponent being tensioned with the help of a tensioning devicecomprising tensioning members (32) arranged at said feed end, each saidtensioning member being flexibly connected by an extension member (34)to a pulling member (35) which is guided in a predetermined pullingdirection by guides (37, 38) and is fixedly connected to a tensioningelement (24) which is hooked on to a hooking member (20) at the end ofsaid screening element (18).
 5. A vibrating screen in accordance withclaim 4, wherein a flexible connection member (39) which transmitstensioning force from said extension member (34) to said pulling member(35) is located lower than a point of engagement between said tensioningelement (24) and said hooking member. (20) and is provided with a guidemember (36) engaging said guides (37, 38), said tensioning force therebyresulting in a jamming effect between said guide member and guides.
 6. Avibrating screen in accordance with claim 4 wherein the tensioningmembers are tensioning screws.
 7. A vibrating screen in accordance withclaim 1, wherein at said feed end (7) of said screen removable covers(40) are provided at each said lower screen deck or decks (11-13)substantially at right angles to the inclination of said decks, saidcovers also supporting spillage protection plates (41) arranged toprevent spillage of material between one end of the respective saidscreening element (17) and said cover.
 8. A vibrating screen inaccordance with claim 1, wherein there are three or more screen decks(11-13).
 9. A vibrating screen in accordance with claim 1, wherein saidcomponent screens of each said screen deck are arranged in steps so thatthe end portions of adjoining component screens are located at avertical distance from each other, a turnover movement thereby beingimparted to the material transported along a said screen deck when saidmaterial is transferred from one component screen to another.
 10. Avibrating screen in accordance with claim 9, wherein said adjoiningcomponent screens of a said screen deck overlap each other a distance inthe horizontal plane.
 11. A vibrating screen in accordance with claim 1,wherein the screening elements are formed of metal wire cloth.
 12. Avibrating screen in accordance with claim 1, wherein the screeningelements are formed of nettings.